1. Field of the Invention
The present invention relates to a liquid crystal display device, and in particular to a liquid crystal display device of a transflective type which is capable of displaying in a transmission mode and displaying in a reflection mode.
2. Description of the Related Art
In recent years, portable electronic devices such as mobile phones and PDAs (Personal Digital Assistants) have been in wide use. In a display section of a portable electronic device, a liquid crystal display device is frequently used because of its advantages in terms of thinness, light weight, and low power consumption.
A liquid crystal display device is not a self-light-emitting type display device such as CRTs and PDPs (plasma display panels). Therefore, in a transmission-type liquid crystal display device, an illuminator called a backlight is provided at the rear surface side of the liquid crystal display panel, and an image is displayed as the transmitted amount of the illumination light from this backlight is controlled by the liquid crystal display panel in a pixel-by-pixel manner.
Liquid crystal display devices of various methods are known. However, some methods (e.g., methods using a TN type or STN type liquid crystal display element) have a disadvantage of narrow viewing angles, and various techniques are under development for overcoming this disadvantage.
As a representative technique for improving the viewing angle characteristics of a liquid crystal display device, there is a method of adding an optical compensation plate. There is also known a method of enhancing the directivity (degree of parallelism) of light which is emitted from a backlight before the light enters a liquid crystal display element, and allowing the light having traveled through the liquid crystal display element to be diffused by a lenticular lens sheet which is disposed at the front surface of the liquid crystal display element (e.g., Japanese Laid-Open Patent Publication No. 9-22011).
FIG. 16 shows a liquid crystal display device 800 disclosed in Japanese Laid-Open Patent Publication No. 9-22011. The liquid crystal display device 800 includes a liquid crystal display panel 820 including a liquid crystal layer (not shown), a backlight 810 disposed at the rear surface side of the liquid crystal display panel 820, and a lenticular lens sheet 830 disposed at the viewer's side of the liquid crystal display panel 820.
The backlight 810 includes a light source 801, a light guiding plate 802 for guiding the light having been emitted from the light source 801 to the liquid crystal display panel 820, and a reflective layer 804 for reflecting the light leaking from the light guiding plate 802 toward the light guiding plate 802. The light guiding plate 802 has an outgoing surface through which light goes out toward the liquid crystal display panel 820, and a rear surface opposing the outgoing surface. A plurality of prisms 802a are provided on the rear surface thereof.
While propagating within the light guiding plate 802, the light having been emitted from the light source 801 is reflected toward the liquid crystal display panel 820 by the prisms 802a on the rear surface, so as to go out through the outgoing surface. Each prism 802a on the rear surface has two slopes that are slanted at respectively different predetermined angles with respect to the outgoing surface, so that the light which is emitted from the backlight 810 has a very strong intensity along the display surface normal direction (frontal direction). In other words, a high directivity is imparted to the light emitted from the backlight 810.
When the light emitted from the backlight 810 has a high directivity, the light traveling through the liquid crystal layer of the liquid crystal display panel 820 can be uniformly modulated (i.e., a uniform retardation can be imparted to the light traveling through the liquid crystal layer). Therefore, the viewing angle dependence of display quality associated with the refractive index anisotropy of liquid crystal molecules can be reduced. As it is, the light having traveled through the liquid crystal display panel 820 has a high directivity, and has a large imbalance in luminance (a very high luminance exists along the display surface normal direction whereas luminances along oblique directions are low). However, through diffusion by the lenticular lens sheet 830, which is a light diffusing element, the luminance imbalance is reduced, and the viewing angle is broadened. As a result, the liquid crystal display device 800 realizes displaying with a wide viewing angle, with a reduced viewing angle dependence in display quality.
On the other hand, in recent years, a liquid crystal display device which is capable of performing high-quality display both outdoors and indoors has been proposed (e.g., Japanese Laid-Open Patent Publication No. 11-101992). This liquid crystal display device is capable of displaying in a transmission mode by utilizing light from a backlight and displaying in a reflection mode by utilizing ambient light (external light), and is called a transflective-type liquid crystal display device.
A transflective-type liquid crystal display device has both of the following features: the low power consumption of a reflection-type liquid crystal display device and the bright and high-contrast-ratio displaying of a transmission-type liquid crystal display device which is free from influences of ambient brightness. Furthermore, it also improves the weakness of a transmission-type liquid crystal display device, i.e., deterioration in visual recognition in a very bright environment of use (e.g., outdoors in fine weather).
However, the transflective-type liquid crystal display device disclosed in Japanese Laid-Open Patent Publication No. 11-101992 lacks a light diffusing element at the viewer's side of the liquid crystal layer, and thus is incapable of performing display with a wide viewing angle. Therefore, it might be conceivable to apply the approach of using a light diffusing element as disclosed in Japanese Laid-Open Patent Publication No. 9-22011 to a transflective-type liquid crystal display device.
However, even if the approach disclosed in Japanese Laid-Open Patent Publication No. 9-22011 is applied to a transflective-type liquid crystal display device as it is, it is difficult to perform high-quality displaying in both the transmission mode and the reflection mode. This is because, when simply a light diffusing element is provided in a transflective-type liquid crystal display device, light which is used for displaying in the transmission mode (light from a backlight) travels through the light diffusing element only once whereas light which is used for displaying in the reflection mode (light from the viewer's side) travels through the light diffusing element twice.
For example, when the haze value (the degree to which light is diffused) of the light diffusing element is optimized for the light which is used for displaying in the transmission mode, the light which is used for displaying in the reflection mode is excessively diffused while traveling through the light diffusing element twice, thus resulting in a dark displaying in the reflection mode.
On the other hand, if the haze value of the light diffusing element is set to be weaker so as not result in a dark displaying in the reflection mode, the light which is used for displaying in the transmission mode travels through the light diffusing element only once and therefore is not sufficiently diffused. As a result, the viewing angle cannot be sufficiently broadened with respect to displaying in the transmission mode.